The present invention relates to a method of producing an electrolytic capacitor with an aluminum-titanium alloy anode body.
A porous sintered body for an electrolytic capacitor using an aluminum-titanium alloy as the anode material is disclosed in U.S. Pat. No. 4,331,477, which is owned by the assignee of the present invention, the patent having issued on May 25, 1982. A porous Al-Ti alloy body can be obtained by the following steps, for example. First, aluminum and either titanium or titanium hydride (TiHz) in the form of powder in the micron order are sufficiently mixed. The mixed powder is then press-molded with a lead wire embedded in it. The mold is sintered in a vacuum of 1.times.10.sup.-6 mm Hg, at a temperature of 1,000.degree. to 1,100.degree. C., for 1 to 3 hours, to obtain an alloy porous sintered body. An Al-Ti alloy reaction occurs during sintering. Aluminum is exclusively diffused into and absorbed by titanium, so that the portions where aluminum has been previously present become and remain porous, thereby providing the porous alloy body. The method of providing an electrolytic capacitor with Al-Ti alloy anode is disclosed in the copending application Ser. No. 509,153 filed on June 28, 1983.
The Al-Ti alloy having the excellent capacitor characteristics has also the characterizing feature that a porous body can be easily obtained having a large specific surface area.
In order to make a solid electrolytic capacitor, the Al-Ti alloy is subjected to an anodic oxidation to form an oxide film by using an aqueous solution of phosphoric acid. Thereafter, a manganese dioxide (MnO.sub.2) layer is formed by a thermal decomposition of manganese nitrate. Then graphite and silver-paste layers are formed on the MnO.sub.2 layer by a process which is essentially the same as the process used for forming conventional Ta capacitors.
The temperature of the heat treatment of the thermal decomposition of the manganese nitrate generally ranges between 200.degree. C. and 250.degree. C. At such a high temperature, the oxide film is thermally deteriorated.
It was found that the oxide film was deteriorated much more remarkably on the thermal decomposition of manganese nitrate than it was deteriorated in the case of a Ta capacitor. It is thought that this deterioration occurs because both Ti and Al essentially are far more chemically active than Ta.
When the oxide film is thermally deteriorated, dielectric characteristics such as the dielectric loss and the temperature coefficient of the electrostatic capacitance are increased.